With all the hype around the impending era of drone delivery, the industry is grappling with questions like air traffic management, pickup and drop-off locations, and security. There’s the debate over whether drones are more or less cost efficient than traditional postal trucks.

But one of the questions that the industry has only scratched the surface on: are drones more environmentally friendly than parcel delivery trucks?

The short answer is: sometimes. And here’s the long answer:

On the surface, drones create less carbon pollution than trucks. Most drones are battery powered, and can be recharged through green energy sources like solar power. There is no gasoline involved or exhaust produce from delivery trucks.

But delivery trucks can also offer a massive amount of packages in one trip, while a drone can only transport small payloads at a time.

Moving ALL Amazon deliveries to drones would be the equivalent of running approximately 3-5x as many vans on the road, according to iniLabs CEO Kynan Eng. But most delivery companies are pushing drones for either “last-mile deliveries” or for extremely lightweight deliveries.

Impact of drones vs. trucks on carbon pollution

UW civil and environmental engineering graduate student Jordan Toy analyzed various real world scenarios to estimate carbon dioxide emissions for a paper published in Transportation Research Part D.

Toy created a heat map to show carbon dioxide emission differences between drone and truck deliveries as a drone’s energy requirements, which are measured in watt-hours per mile and the number of stops on a route increase. Red areas reflect conditions in which drones emit less carbon dioxide than trucks (lighter packages, fewer stops), while blue areas denote conditions in which drones emit more (heavier packages, more stops).

In a nutshell, small, light packages are very environmentally friendly from a carbon emissions standpoint when delivered by drones, but once the delivery route adds more stops or runs farther out from the warehouse, it becomes less environmentally friendly.

Impact of drones on wildlife

But it’s not all about carbon pollution. There are other environmental factors at stake.

That being said, cars aren’t exactly friendly to animals. An estimated 1.25 million insurance claims are filed annually due to vehicle collisions with large animals, while building roads can cause habitat destruction or fragmentation.

It seems the consensus is that drone delivery could be useful for last-mile deliveries, helping a central warehouse get items out in 30 minutes or less to customers who live in the same city.

Or as Eng puts it: “Under certain circumstances, if one insists on drone delivery it may be most efficient to have a giant drone carrier hovering constantly above a city, similar to that seen in the Avengers movie franchise. Or not.”